This invention relates to magnetic recording media, such as thin film magnetic recording disks, and to a method of manufacturing the media. The invention has particular applicability to high areal density longitudinal magnetic recording media having very low medium noise and high degree of thermal stability, and more particularly, to a laminated medium with antiferromagnetic stabilization layers.
The increasing demands for higher areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of remanent coercivity (Hr), magnetic remanance (Mr), coercivity squareness (S*), signal-to-medium noise ratio (SMNR), and thermal stability of the media. In particular, as the SMNR is reduced by decreasing the grain size or reducing exchange coupling between grains, it has been observed that the thermal stability of the media decreases. Therefore, it is extremely difficult to produce a magnetic recording medium satisfying above mentioned demanding requirements.
Magnetic discs and disc drives provide quick access to vast amounts of stored information. Both flexible and rigid discs are available. Data on the discs is stored in circular tracks and divided into segments within the tracks. Disc drives typically employ one or more discs rotated on a central axis. A magnetic head is positioned over the disc surface to either access or add to the stored information. The heads for disc drives are mounted on a movable arm that carries the head in very close proximity to the disc over the various tracks and segments.
FIG. 1 shows the schematic arrangement of a magnetic disk drive 10 using a rotary actuator. A disk or medium 11 is mounted on a spindle 12 and rotated at a predetermined speed. The rotary actuator comprises an arm 15 to which is coupled a suspension 14. A magnetic head 13 is mounted at the distal end of the suspension 14. The magnetic head 13 is brought into contact with the recording/reproduction surface of the disk 11. A voice coil motor 19 as a kind of linear motor is provided to the other end of the arm 15. The arm 15 is swingably supported by ball bearings (not shown) provided at the upper and lower portions of a pivot portion 17.
A cross sectional view of a conventional longitudinal recording disk medium is depicted in FIG. 2. A longitudinal recording medium typically comprises a non-magnetic substrate 20 having sequentially deposited on each side thereof an underlayer 21, 21xe2x80x2, such as chromium (Cr) or Cr-containing, a magnetic layer 22, 22xe2x80x2, typically comprising a cobalt (Co)-base alloy, and a protective overcoat 23, 23xe2x80x2, typically containing carbon. Conventional practices also comprise bonding a lubricant topcoat (not shown) to the protective overcoat. Underlayer 21, 21xe2x80x2, magnetic layer 22, 22xe2x80x2, and protective overcoat 23, 23xe2x80x2, are typically deposited by sputtering techniques. The Co-base alloy magnetic layer deposited by conventional techniques normally comprises polycrystallites epitaxially grown on the polycrystal Cr or Cr-containing underlayer.
A conventional longitudinal recording disk medium is prepared by-depositing multiple layers of films to make a composite film. In sequential order, the multiple layers typically comprise a non-magnetic substrate, one or more underlayers, one or more magnetic layers, and a protective carbon layer. Generally, a polycrystalline epitaxially grown cobalt-chromium (CoCr) alloy magnetic layer is deposited on a chromium or chromium-alloy underlayer.
Conventional methods for manufacturing a longitudinal magnetic recording medium with a glass, glass-ceramic, Al or Alxe2x80x94NiP substrate may also comprise applying a seedlayer between the substrate and underlayer. A conventional seedlayer seeds the nucleation of a particular crystallographic texture of the underlayer. Conventionally, a seedlayer is the first deposited layer on the non-magnetic substrate. The role of this layer is to texture (alignment) the crystallographic orientation of the subsequent Cr-containing underlayer, and might also produce small grain size, which is desired for the purpose of reducing recording noise.
The seedlayer, underlayer, and magnetic layer are conventionally sequentially sputter deposited on the substrate in an inert gas atmosphere, such as an atmosphere of argon. A conventional carbon overcoat is typically deposited in argon with nitrogen, hydrogen or ethylene. Conventional lubricant topcoats are typically about 20 xc3x85 thick.
A substrate material conventionally employed in producing magnetic recording rigid disks comprises an aluminum-magnesium (Alxe2x80x94Mg) alloy. Such Alxe2x80x94Mg-alloys are typically electrolessly plated with a layer of NiP at a thickness of about 15 microns to increase the hardness of the substrates, thereby providing a suitable surface for polishing to provide the requisite surface roughness or texture.
Other substrate materials have been employed, such as glass, e.g., an amorphous glass, glass-ceramic material that comprises a mixture of amorphous and crystalline materials, and ceramic materials. Glass-ceramic materials do not normally exhibit a crystalline surface. Glasses and glass-ceramics generally exhibit high resistance to shocks.
According to the domain theory, a magnetic material is composed of a number of submicroscopic regions called domains. Each domain contains parallel atomic magnetic moments and is always magnetized to saturation (Ms), but the directions of magnetization of different domains are not necessarily parallel. In the absence of an applied magnetic field, adjacent domains may be oriented randomly in any number of several directions, called the directions of easy magnetization, which depend on the geometry of the crystal, stress, etc. The resultant effect of all these various directions of magnetization may be zero, as is the case with an unmagnetized specimen. When a magnetic field is applied, the domains most nearly parallel to the direction of the applied field may grow in size at the expense of the others. This is called boundary displacement of the domains or the domain growth. Domains may also rotate and align parallel to the applied field. When the material reaches the point of saturation magnetization, no further domain growth and rotation would take place on increasing the strength of the magnetic field.
The ease of magnetization or demagnetization of a magnetic material depends on the crystal structure, grain orientation, the state of strain, and the direction of the magnetic field. The magnetization is most easily obtained along the easy axis of magnetization but most difficult along the hard axis of magnetization. A magnetic material is said to posses a magnetic anisotropy when easy and hard axes exist. On the other hand, a magnetic material is said to be isotropic when there are no easy or hard axes. A magnetic material is said to possess a uniaxial anisotropy when the easy axis is oriented along a single crystallographic direction, and to possess multiaxial anisotropy when the easy axis aligns with multiple crystallographic directions.
xe2x80x9cAnisotropy energyxe2x80x9d is the work against the anisotropy force to turn magnetization vector away from an easy direction. For example, a single crystal of iron, which is made up of a cubic array of iron atoms, tends to magnetize in the directions of the cube edges along which lie the easy axes of magnetization. A single crystal of iron requires about 1.4xc3x97105 ergs/cm3 (at room temperature) to move magnetization into the hard axis of magnetization from an easy direction, which is along a cubic body diagonal.
The anisotropy energy UA could be expressed in an ascending power series of the direction cosines between the magnetization and the crystal axes. For cubic crystals, the lowest-order terms take the form of Equation (1),
UA=K1(xcex112xcex122+xcex122xcex132+xcex132xcex112)+K2(xcex112xcex122xcex132)xe2x80x83xe2x80x83(1)
where xcex11, xcex12 and xcex13 are direction cosines with respect to the cube, and K1, and K2 are temperature-dependent parameters characteristic of the material, called anisotropy constants.
Anisotropy constants can be determined from (1) analysis of magnetization curves, (2) the torque on single crystals in a large applied field, and (3) single crystal magnetic resonance. The term xe2x80x9canisotropy constantxe2x80x9d is often referred to as magnetocrystalline anisotropy constant.
While Equation (1) applies for a cubic lattice, similar equations are also known for other lattice systems. For example, for a hexagonal close packed (HCP) lattice, the equation for UA is the following:
UA=K1 sin2xcex8+K2 sin4xcex8xe2x80x83xe2x80x83(2)
where xcex8 is the angle between the Ms vector, i.e., the saturation magnetization direction, and the c axis (easy axis), and K1 and K2 are anisotropy constants.
The requirements for high areal density, i.e., higher than 30 Gb/in2, impose increasingly greater requirements on magnetic recording media in terms of coercivity, remanent squareness, medium noise and track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements, particularly a high-density magnetic rigid disk medium for longitudinal and perpendicular recording. The magnetic anisotropy of longitudinal and perpendicular recording media makes the easily magnetized direction of the media located in the film plane and perpendicular to the film plane, respectively. The remanent magnetic moment of the magnetic media after magnetic recording or writing of longitudinal and perpendicular media is located in the film plane and perpendicular to the film plane, respectively.
Important magnetic properties, such as coercivity (Hc), remanent magnetization (Mr) and coercive squareness (S*), which are crucial to the recording performance of the Co alloy thin film for a fixed composition, depend primarily on its microstructure. For thin film longitudinal magnetic recording media, the desired crystalline structure of the Co and Co alloys is HCP with uniaxial crystalline anisotropy and a magnetization easy direction along the c-axis is in the plane of the film. The better the in-plane c-axis crystallographic texture, the higher the coercivity of the Co alloy thin film used for longitudinal recording. This is required to achieve a high remanence. For very small grain sizes coercivity increases with increased grain size. As grain size increases, noise increases. There is a need to achieve high coercivities without the increase in noise associated with large grains. To achieve a low noise magnetic medium, the Co alloy thin film should have uniform small grains with grain boundaries that can magnetically isolate neighboring grain diameters. This kind of microstructure and crystallographic texture is normally achieved by manipulating the deposition process, or most often by the proper use of an underlayer.
The linear recording density can be increased by increasing the Hr of the magnetic recording medium, and by decreasing the medium noise, as by maintaining very fine magnetically non-coupled grains. Medium noise in thin films is a dominant factor restricting increased recording density of high-density magnetic hard disk drives, and is attributed primarily to inhomogeneous grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, medium noise must be minimized by suitable microstructure control and, therefore, several techniques have been disclosed in the prior art.
For example, U.S. Pat. No. 5,462,796 (Teng) teaches a laminated longitudinal magnetic recording medium with Cr-containing non-magnetic layer between two magnetic layers. This medium exhibits a lower medium noise than that of a conventional medium without the Cr-containing interlayer. However, when the medium Mrt is below 0.6 memu/cm2, the laminated medium has very poor thermal stability, which will be shown below. As recording density increases to about 30 Gb/in2, medium Mrt has been reduced to about 0.35 memu/cm2. Regular laminated medium can not be used in such low Mrt regime due to thermal stability issue.
Abarra et al. (xe2x80x9cLongitudinal magnetic recording media with thermal stabilizationxe2x80x9d, AA-06, Intermag 2000 Digest of Technical papers, 2000 IEEE International Magnetics Conference, Toronto, Canada, Apr. 9-13, 2000.) reported the approach to insert a Ru film about 8 xc3x85 thick into two ferromagnetic layers to enhance the thermal stability of the recording layer, located on the top of Ru film.
CoCr films with Cr concentration around 37 atomic percent are non-magnetic films with hexagonal structure. Ohkijima et al. (xe2x80x9cEffect of CoCr interlayer on longitudinal recording,xe2x80x9d IEEE Transactions on Magnetics, Vol. 33, No. 5, pp. 2944-2946, September 1997) reported the use of CoCr layer deposited between Cr underlayer and CoCrTa magnetic layer.
In order to squeeze as much digital information as possible on a recording disc medium there is a need to find a film structure, which can benefit the low noise feature of laminated medium, but has acceptable thermal stability. Furthermore, in order to obtain high enough signal output, and reduce the medium noise of the medium with antiferromagnetic stabilization layers, further improvement of the medium is necessary.
Applicants recognized that the antiferromagnetic interactions of two Co-containing magnetic layers, one of the Co-containing magnetic layers being a magnetic recording layer and the other being a stabilization layer, separated by a thin Ru film magnetically stabilize the magnetic recording layer. Applicants also recognized that the use of a CoCr-containing non-magnetic hexagonal-structured film in between two Co-containing magnetic layers provides decoupling and improved epitaxial growth of the Co-containing magnetic layers.
The invention provides a magnetic recording medium for high areal recording density exhibiting low noise and high degree of thermal stability. One way of achieving this goal is to produce a magnetic recording medium having (1) a non-magnetic layer, preferably, a Ru-containing layer, between two Co-containing magnetic layers, one of the Co-containing magnetic layers being a magnetic recording layer and the other being a stabilization layer, and (2) a non-magnetic interlayer, preferably, a CoCr-containing non-magnetic hexagonal-structured layer, in between two Co-containing magnetic recording layers.
An embodiment of this invention is a magnetic recording medium, comprising a Co-containing recording layer and an additional Co-containing recording layer separated by a non-magnetic interlayer, and further comprising a Ru-containing layer and a Co-containing stabilization layer, wherein the magnetic recording medium is thermally stable. The magnetic recording medium could further comprise an additional Ru-containing layer and an additional Co-containing stabilization layer. The Co-containing recording layer and the additional Co-containing recording layer each could have n pairs of the Ru-containing layer and the Co-containing stabilization layer deposited immediately under the recording layer, wherein n is more than 1. The Ru-containing layer could have a thickness in a range of about 0.6 to 1.2 nm. The non-magnetic interlayer could comprise a hexagonal-structured non-magnetic film having a similar lattice constant as that of the Co-containing recording layer. The non-magnetic interlayer could comprise Cr and X, wherein the X is selected from the group consisting of V, Mo, W, Ti, Ru and RuW and the X is capable of expanding the lattice constant of the non-magnetic interlayer. The thickness of the non-magnetic interlayer could be in a range of about 0.5 to 5 nm. The Co-containing stabilization layer could comprise a Co-containing alloy selected from the group consisting of CoCrPt, CoCrPtTa, CoCrPtTaNb and CoCrPtB. The Co-containing stabilization layer could have a thickness in a range of about 1 to 5 nm. The Co-containing recording layer and/or the additional Co-containing recording layer could comprise a Co-containing alloy selected from the group consisting of CoCrPt, CoCrPtTa, CoCrPtTaNb, and CoCrPtB, and have a thickness in a range of about 4 to 12 nm.
Another embodiment of this invention is a method of manufacturing a magnetic recording medium comprising, depositing a Co-containing stabilization layer on a Cr-containing underlayer, depositing a Ru-containing layer on the Co-containing stabilization layer, depositing a Co-containing recording layer on the Ru-containing layer, depositing a non-magnetic interlayer on the Co-containing recording layer, and depositing an additional Co-containing recording layer on the non-magnetic interlayer.
Yet, another embodiment is a magnetic recording medium, comprising a pair of magnetic recording layers separated by means for improving thermal stability of the magnetic recording medium. In this invention, xe2x80x9cmeans for improving thermal stability of the magnetic recording mediumxe2x80x9d is a combination of a Ru-containing layer and a Co-containing layer, a combination of a Ru-containing layer and a CoCr-containing non-magnetic layer or combinations thereof.
As will be realized, this invention is capable of other and different embodiments, and its details are capable of modifications in various obvious respects, all without departing from this invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not as restrictive.